1. Field of the Invention
This invention relates to a filter holder which may be spectacle mounted that rotates a round variable neutral density filter and thus reduces the contrast of various test objects viewed through the filter.
2. Description of Prior Art
In 1947 an ophthalmologist, named Amsler, described a grid pattern to evaluate central visual fields. Since that time various investigators have modified the grid so that the contrast of the background and grid can be varied to make the test much more sensitive to relative losses in field. Miller, Lamberts and Perry described an illuminated grid in Archives of Ophthalmology 1978; 96 901-902. Bernth-Petersen described a transilluminated Amsler grid in Acta Ophthalmology 1981, 59: 57-63. Finally, Wall and Sudun suggested using two rotating polaroid filters to attenuate the Amsler grid in Archives of Ophthalmology 1986 104: 520-523.
All of these devices are limited in their success because levels of density and contrast are variable and not properly calibrated and controlled. By using a variable neutral density filter the exact transmittance of the filter is known. It can be read directly from the scale on the device which reads transmittance as a function of the rotation of the control knob.
The object of the present invention is to provide a device which overcomes the deficiencies of prior art and enables the transmittance to be read directly without having to calibrate each device, each time it is used.